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Old 01-08-2011, 05:00 PM   #21
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Thanks for the info. I think that in my health condition (old and fat and bad out of shape) the trailer-aid is going to become standard equipment.
Mickey
Since I am in that camp (OF&BOOS), soon I will be depending on AAA for my ALL my roadside service and screw that jack thingy. I will only change tires on the curb side of the road in any case. I leave the car zipping past side to the professionals.
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Old 01-09-2011, 08:39 AM   #22
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If you use one of the drive-on jacking aids, that puts all of the weight on one axle. We only have 4000# rubber axles, so that would overload the one by a factor of 50%. While that probably wouldn't hurt a regular spring-type suspension, I wonder if it would damage the rubber axle?
I suppose it shouldn't, as shock loads while driving are probably far higher than a static overload, but it does make me ponder.
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Old 01-09-2011, 09:13 AM   #23
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If you use one of the drive-on jacking aids, that puts all of the weight on one axle. We only have 4000# rubber axles, so that would overload the one by a factor of 50%.
John,

While on the surface that might sound logical, it not really true.

With 4 wheels on the ground the CG is calculated by drawing lines that connect each tire (forming a rectangle). The CG is in the exact center of the rectangle. This can be easily found by drawing an X through the 4 wheels. Since the intersection is equidistant from each wheel, each wheel is carrying an equal amount of weight. (ie 1/4th of the total load)

With 3 on the ground the actual load on each wheel gets a bit more complicated. We need to find out the new CG location to determine the actual load on the wheels in contact with the ground. To do this lines are drawn connecting the wheels as before, but it now forms a right triangle with an 8 foot side and a side that connects the centers of the tandem.
Use geometry to determine the center of the triangle (draw lines bisecting the angles of the triangle and the CG is the intersecting point). The actual load on each wheel is then calculated using vectors to each wheel. If the 3 wheels are equidistant apart (8 foot x 8 foot say), the weight on each wheel would be exactly 1/3 of the total camper weight. Since they are not, the load of the missing wheel is shared unequally by the 3 remaining tires. (slightly more on the side with 2 tires - shared by the 2 tires equally; slightly less on the side with one tire)

While the tires are still "overloaded" per se, that load rating is calculated to be the maximum load at 60 MPH and 50 PSI (for load range C); not a static load. This is well within the capability of the tire for brief periods.

That is why you can drive on a blowout to a safe spot without immediately blowing all the others (just not at 75 MPH!). The slower as safely possible the better.
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Old 01-09-2011, 10:41 AM   #24
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Lou, I think we had this discussion on another thread, and I am still not convinced that the weight is equally distributed (or close) between the 3 tires left after a flat tire. Your explanation is thorough, but I just can't get it through my thick skull.

It seems to me that during a flat, the CG of the camper does not change......it is centered still forward and center of the axles just like it is when all 4 tires are supporting the weight. Using the bisecting lines at the vectors will give you the center of the triangle, which would indicate equal weight on all 3 of the remaining tires. But the CG of the camper itself shouldn't change......other than leaning towards the flat side which would actually add more weight to that side.

I am so confused.
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Old 01-09-2011, 10:50 AM   #25
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mtnguy is correct.
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Old 01-09-2011, 10:54 AM   #26
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Since most of us who have actually changed a flat (pretty close to 100% of us with campers here) do so when still hooked up, the whole dynamic is even more confusing. I think we are just going to have to say...

The tire is safe to support however you decide to change it. Whether 25% of the supported weight; 33% of the supported weight; 50% of the supported weight, or some bizzaro "other number"; as long as you slow down, pull over, and safely support the camper while changing the tire you will be safe. Changing the "other tire on that side" as some have suggested is not neccessary unless you suspect the reason for failure to be tire age or dry rot; then change them ALL.
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Old 01-10-2011, 10:32 AM   #27
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Bumped this whole thread to my sister. She works at Lockheed space and is a "no kidding" rocket scientist. She works in reentry ballistics (nuclear weapon) area but knows a bit about mechanical engineering since she has that degree. No answer from her yet this morning, I am sure she is working on her latest Reagan Missile test center results. Will let you know her reply.

I did find this though and, while it sounds like double speak, is kinda what I was trying to explain.

Answers.com - How do you calculate the bending moments for a uniformly distributed loads with point loads
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Old 01-10-2011, 10:54 AM   #28
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It actually does not require a 'rocket scientist" to figure it out.
The dry axle weight on my fifth is about 6200 lbs, 3100 lbs per side. Raise one wheel off the ground, and there is still 3100 lbs on that side. Nothing changes, except that there is now 3100 lbs being supported by one wheel instead of two. Thus my concern considering the axle rating for that one wheel is only 2000 lbs on my 4000 lb rated rubber axles.
I am also an engineer, top 3% of my class of '77.
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Old 01-10-2011, 11:31 AM   #29
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You are probably right and I am "overthinking" it.
Seems counter-intuitive to me though.
Why it would not be 1/3 of the weight still escapes me.
I trust my sister's answer (when ever I get it) as gospel.
She got the looks and the brains in our family.
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Old 01-10-2011, 11:52 AM   #30
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Just think of it this way, if you took both wheels off the one side, would there be no weight there?
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